1. A method for identifying fault types of an HVDC physical transmission line, including the following steps:(1) measuring positive and negative voltage values ujp, ujn, ukp, ukn and positive and negative current values ijp, ijn, ikp, ikn at j-terminal and a k-terminal of the HVDC physical transmission line with a physical voltage meter and a physical current meter, and converting the measured voltage and current values under the maximum amount to voltage and current values uj0, uj1, uk0, uk1, ij0, ij1, ik0, ik1 under mode components by employing decoupling matrix with a first convertor embedded in a first circuitry;

(2)calculating, by a processing system, the differential current values dIj0(t), dIj1(t), dIk0(t), dIk1(t) of travelling wave at the j-terminal and k-terminal under zero-mode and one-mode components of the HVDC physical transmission line, respectively;

(3)comparing with the calculated differential current value of the travelling wave at the one-mode components under the current time t and the same at time t-t0 based on the following criteria:

dIj1(t)>K·dIj1(t?t0) or dIk1(t)>K·dIk1(t?t0)

Where k is a reliability coefficient, the value of which is between 1.5 and 2; t0 represents the length of time to select data from current time; and it is determined that the HVDC physical transmission line is faulty if the criteria are satisfied at three consecutive sampling points, and then the method proceeds to the next step;

(4)converting the differential current values of the travelling wave at the j-terminal and k-terminal under the mode components to differential current values dIjp(t), dIjn(t), dIkp(t), dIkn(t)under the maximum amount, respectively with a second convertor embedded in a second circuitry;

(5)calculating, by the processing system, fault type coefficients of both terminals of the HVDC physical transmission line, respectively:

(6)setting a small positive number a, and identifying, by the processing system, the fault types based on the fault type coefficients:

1 if the absolute value of the fault type coefficient at the j-terminal or k-terminal is less than a, identifying, by the processing system, a fault type is pole-to-pole fault;

2 if the absolute value of the fault type coefficient at the j-terminal or k-terminal is greater than a, identifying, by the processing system, the fault type is positive-pole-to-ground fault;

3 if the absolute value of the fault type coefficient at the j-terminal or k-terminal is less than ?a, identifying, by the processing system, the fault type is negative-pole-to-ground fault.